I propose to use the two-microelectrode technique of voltage-clamping the sleep cardiac Purkinje fiber to examine in detail how cardiac contraction depends on membrane voltage, ionic currents, and time. The principal goal of the project is to examine the behavior of the slow inward current (Isi) and its relation to contraction. Specific areas of investigation include: attempts to separate clearly the slow inward current from the transient outward (Iqr), which obscures Isi at voltages positive to -20 mV; analysis of the kinetics of Isi and Iqr; comparisons of the kinetics of Isi to those of contraction; and experiments designed to examine the factors governing the rate of recovery of the contractile system in heart.